The essence of the problem is that the gallery is functioning really badly and people on the forum don't even reply anymore to questions for help. But there are many posts just like yours so you will find a lot of info on the forum.

Size of pictures is important. You cannot upoad more than a few megabytes at once.

Other thing is that the login for the gallery ds different from your kitmaker login so you have to make a special gallery account.

The good thing is that the siteowner promised a solution for all this soon.